The present application relates to thermally insulated containers, and, more particularly, to thermally insulated containers which use vacuum panels as a primary mechanism to avoid thermal loss. Such thermally insulated containers can be used for maintaining food, drink or medical items in a cold or frozen state without an outside energy or cooling source.
Containers such as coolers have long been used to thermally insulate hot items or frozen or refrigerated items. Many items which are frozen or refrigerated are perishables such as food items which must be maintained at a cold or frozen temperature to satisfactorily inhibit bacteria growth. The coolers typically contain walls made out of a thermally insulated material, such as a closed cell foam (for example, STYROFOAM) or other thermally insulating material. For repeated use in conjunction with food items, the thermal insulation layer is commonly housed in a more durable, sanitary housing structure, such as plastic, aluminum or stainless steel sheet material as layers on the inside and/or outside of the thermal insulation layer. Such coolers usually include a relatively flat base, generally vertical peripheral walls, and a removable lid which together form an enclosure. Each of the base wall, peripheral walls and lid may be thermally insulated.
The coolers are generally wide-mouthed, with the lid being approximately the same size as the base, with the lid extending across the wide mouth. With the wide-mouthed construction, items placed in the cooler may be as large as the insulated chamber, because no neck is present to interfere with placement or removal of the items into or out of the cooler.
In some instances the thermal insulation layer is provided by a vacuum between two spaced wall layers. For instance, vacuum insulated containers may come in the form of a circularly drawn vacuum bottle. Vacuum bottles are usually constructed with a small opening or neck, and are intended for holding liquid. Vacuum bottles are not commonly used to hold solid items such as perishable food items, because the neck is too small for the food items to pass.
Vacuum insulation has also been available in a second form, as planar vacuum panels. A container constructed of planar vacuum panels would likely include six separate side walls joined to form a cubical or box shape, including twelve edges connected between the six sides of the vacuum panels. Such containers have a primary thermal difficulty, referred to as "edge loss", which must be overcome. In particular, while the panels themselves are very efficient thermal insulators, the edges between panels can contribute to thermal losses which are more significant than the thermal efficiency provided by the panels themselves. Because of edge loss problems and cost of manufacture, vacuum panels have not gained widespread acceptance for use in container walls.
It has also been long recognized that the thermal insulation provided by coolers may not always be sufficient to maintain the cold state of a product over a prolonged period of time. For this reason, various coolant materials have been used in conjunction with the thermally insulated containers. The most basic and common coolant material is ice, which melts at 32.degree. F. or 0.degree. C. with a latent heat of fusion of approximately 80 cal/g, or approximately 333 kJ/kg. The melting phase change of the ice (i.e., the heat absorbed by the ice during melting) maintains the perishable goods near the melting temperature of ice.
One shortcoming of ice is that the result of the phase change is water, and many of the frozen or refrigerated goods should be maintained in a dry state and not exposed to contact with water. Other coolant materials may be poisonous or have harmful effects if ingested, making it even more important that the coolant material does not contact a food item. For this reason, water and other water-based coolant materials have been enclosed in various coolant packets, such as rigid or semi-rigid plastic containers. Another shortcoming of ice is that ice melts at a temperature which is too high to maintain most food items in a frozen state. Thus, ice is a suitable coolant material for refrigerated goods, but not for frozen goods.
Frozen carbon dioxide, or "dry ice", is a commonly used coolant material for frozen goods. Dry ice has a higher latent heat, and a lower phase change temperature than water. Carbon dioxide undergoes a phase change from solid to gas at approximately -78.5.degree. C. or -110.degree. F., with a latent heat of sublimation of about 573 kJ/kg. Skin contact to dry ice is somewhat hazardous, and dry ice should generally be handled without skin contact.
Regardless of the use of coolant materials, the various shortcomings of suitable thermally insulative containers have limited their use in many potential applications. Additional mechanical or thermal means of cooling (i.e., freezers, refrigerated trucks and box cars, etc.), at a significant expense, are often required for handling of frozen items. Additional methods are needed for the handling of frozen items in a warm or ambient for periods of time ranging from several minutes to hours to several days.